How to generate a new string by combine github action workflow variables

I made it work here using this workflow implementation.

For what I observed, there are 2 issues in your implementation:

First issue

According to the documentation, when setting an environment variable, you can't access the variable in the same step.

You can make an environment variable available to any subsequent steps in a workflow job by defining or updating the environment variable and writing this to the GITHUB_ENV environment file. The step that creates or updates the environment variable does not have access to the new value, but all subsequent steps in a job will have access. The names of environment variables are case-sensitive, and you can include punctuation. For more information, see "Environment variables."

Therefore, your implementation should instead looks like this:

- name: Setup env var
  run: |
    echo MESSAGE_SHA='${{ github.event.head_commit.message }}'-'${{ github.sha }}' >> $GITHUB_ENV

- name: Check env var
  run: |
    echo ${{ env.MESSAGE_SHA }}

However, that wouldn't be enough depending the context, due to the second point.

Second issue

The ${{ github.event.head_commit.message }} variable can be a multiline variable, in that case, you would need to create the variable with another syntax before adding it to the $GITHUB_ENV.

The solution I came with was using this implementation:

      - name: Setup env var
        run: |
          MESSAGE=$(cat << EOF
          '${{ github.event.head_commit.message }}'
          -'${{ github.sha }}'
          EOF
          )
          echo MESSAGE_SHA=$MESSAGE >> $GITHUB_ENV

Therefore, combining both concepts above, your workflow implementation should look like this to achieve what you want:

      - name: Setup env var
        run: |
          MESSAGE=$(cat << EOF
          '${{ github.event.head_commit.message }}'
          -'${{ github.sha }}'
          EOF
          )
          echo MESSAGE_SHA=$MESSAGE >> $GITHUB_ENV

      - name: Check env var
        run: |
          echo ${{ env.MESSAGE_SHA }}

Note that using echo MESSAGE_SHA='${{ github.event.head_commit.message }}'-'${{ github.sha }}' >> $GITHUB_ENV could work if you can always guaranty the commit message is a single line.

EDIT:

This syntax also works to set multilines outputs (and looks easier to use):

      run: |
         echo "TEST=first line \
          second line \
          third line" >> $GITHUB_OUTPUT